1. Field of the Invention
The present invention relates to apparatus for controlling the operation of the control loop of a fiber optic gyroscope. More particularly, this invention pertains to apparatus for compensating errors that result from inherent feedback signal time lag.
2. Description of the Prior Art
The Sagnac interferometer is an instrument for determining rotation by measurement of the non-reciprocal phase difference generated between a pair of counterpropagating light beams. This instrument generally comprises a light source such as a laser, an optical waveguide consisting of several mirrors or a plurality of turns of optical fiber, a beamsplitter/combiner, a detector and a signal processor.
In an interferometer, the waves coming out of the beamsplitter counterpropagate along a single optical path. The optical waveguide is "reciprocal"; that is, any distortion of the optical path affects the counterpropagating beams similarly although they do not necessarily experience such perturbation at the same time or in the same direction. Time-varying perturbations may be observed where the time interval is comparable to the propagation time of the light around the optical waveguide whereas "non-reciprocal" perturbations affect the counterpropagating beams differently and according to the direction of propagation. Such non-reciprocal perturbations are occasioned by physical effects that disrupt the symmetry of the optical medium in which the two waves propagate. Two of the non-reciprocal effects are quite well known. The Faraday, or collinear magneto-optic effect, occurs when a magnetic field creates a preferential spin orientation of the electrons in an optical material whereas the Sagnac, or inertial relativistic effect, occurs when rotation of the interferometer with respect to an inertial frame breaks the symmetry of propagation time. The latter effect is employed as the principle of operation of a ring gyroscope.
It is known that the fringe or interference pattern formed by the counterpropagating beams of a gyro consists of two elements, a d.c. component and a component that is related (e.g. cosine function) to the cause of the phase difference between the beams. This phase difference provides a measure of the non-reciprocal perturbation due, for example, to rotation. As a consequence of the shape of the fringe pattern, when small phase differences are to be measured (e.g. low rotation rates), the intensity of the combined beam is relatively insensitive to phase difference as such difference occurs close to the maximum of the phase fringe pattern. Further, mere intensity of the composite beam does not indicate the sense or direction of rotation.
For the foregoing reasons, an artificially biased phase difference is commonly superimposed upon the counterpropagating beams. The biasing of the phase shift, also known as "non-reciprocal null-shift," enhances the sensitivity of the intensity measurement to phase differences. A maximum degree of sensitivity is achieved by shifting the operating point of the gyroscope to .+-..pi./2 (or odd multiples thereof). Furthermore, by alternating the bias between +.pi./2 and -.pi./2, two different operating points are observed. This enables the system to determine the sign of the phase difference and, thus, the direction of rotation.
In addition to phase modulation, the processing of an interferometer output commonly employs "phase nulling" that introduces an additional phase shift through a negative feedback mechanism to compensate for that due to the non-reciprocal (Sagnac) effect. Commonly, the negative feedback generates a phase ramp whose slope is proportional to the rate of rotation to be measured. In actual practice, a ramp whose height varies between 0 and 2 .pi. radians is employed as the nulling phase shift cannot be increased indefinitely due to voltage constraints.
U.S. Pat. No. 4,705,399 of Graindorge et al. discloses a digitally-based arrangement, that employs a "stairstep" waveform. The height of each step is equal to the measured phase difference while the width or period of each is the group delay time of the optical coil. On the average, the slope of the ramp is equivalent to the measured non-reciprocal phase difference per unit of time. This method is compatible with digital signal processing and enjoys many resulting advantages. The phase modulation may be directly added to the digital ramp through the synchronization offered by a digital signal processor. The (combined) signal ultimately controls the phase modulator that is positioned within the optical fiber coil.
The operation of a fiber optic gyroscope accordingly requires the performance of numerous functions, including the above-described modulations (and associated demodulations) on a continuing basis. That is, such actions must be performed generally at least as frequently as each loop transit time .tau.. In the event that additional functions are desired (for example, to increase gyro accuracy), such other functions may also require repetition of actions during each loop transit time.
U.S. Pat. No. 5,337,143 of John G. Mark and Daniel A. Tazartes entitled "Loop Controller For Multiplexed Triaxial Gyro" discloses an application specific integrated circuit ("ASIC") that functions as a loop controller for a triaxial gyro. The controller accepts the digitized outputs of three modulated gyros, measures the rotation associated with each, digitally processes the outputs and provides analog signals for driving the gyro phase modulators. The operations of the loop controller are directed by a microprocessor. As such, a certain degree of flexibility is attained in that various types of modulation (e.g. random, pseudo-random, orthogonal, deterministic) may be input from the microprocessor and the computational power of the microprocessor is available to update system parameters. The loop controller is not programmable in and of itself. As a result, it is limited by both the speed of the associated microprocessor and its own inflexibility. Accordingly, the loop controller is constrained in scope of operation, being essentially limited to the "basic" loop controller functions of gyro modulation, demodulation of rate information, generation of a phase-nulling ramp, outputting of the rate data employed for phase-nulling, resetting of the ramp. It is, of course absolutely essential that the gyro be capable of measuring angular rate and phase modulator scale factor control (to ensure linearity and accurate modulo 2 .pi. operation). Other features, such as loop gain control (for wide bandwidth response) and offset control (to minimize noise), while not essential are extremely desirable capabilities as well.
Pending U.S. patent application Ser. No. 08/520,217 of John G. Mark and Daniel A. Tazartes entitled "Loop Controller For Fiber optic Gyroscope With Distributed Data Processing" discloses a loop controller that employs an architecture that includes distinct units for distributing the necessary data processing functions whereby operations may take place in parallel to enable additional useful functions within each loop transit time. A field programmable gate array generates variables of varying sign while an auxiliary processor updates parameters that do not require updating every loop transit time. The combination of such operations enables elimination of any need for the gyro processor to perform throughput-intensive test and branch operations.
While the prior art discloses a number of loop controller operations, a common problem in the case of closed-loop configurations results from the fact that the feedback signal indicative of the angular rate sensed by the gyro is generated and applied to the phase modulator to maintain a stable operating point. Ideally, the feedback phase should cancel the Sagnac phase to maintain the above-described null condition. Due to the inherent delays present in fiber optic gyro loops (typically two or three times the loop transit time), the feedback signal always lags the Sagnac phase. In the presence of a variable angular rate subject to acceleration and deceleration the feedback phase is continually trying to "catch up" to the actual Sagnac phase leaving a residual instantaneous signal. Such residual signal can become large in the presence of high frequency vibration, saturating the null detection amplifier and the analog-digital converter.